392 



GEOLOGICAL SUCCESSION OF ANIMALS. 



surface have been elevated to great heights, in the form of 

 mountains ; and hence it is that fossils are sometimes found at 

 the summit of the highest mountains, though the rocks con- 

 taining them were originally formed at the bottom of the sea. 

 But even when folded, or partly broken, their relative age may 

 still be determined by an examination of the ends of the up- 

 turned strata, where they appear or crop out in succession, at 

 the surface, or on the slopes of mountains, as seen in the dia- 

 gram (fig. 376). 



Fig. 376. 



649. The sedimentary rocks are the only ones containing 

 animal and vegetable remains. These are found imbedded in 

 the rock, just as we should find them in the mud now deposited 

 at the bottom of the sea, if laid dry. The strata containing 

 fossils are numerous. The comparison and detailed study of 

 them belongs to geology, of which Palaeontology forms an 

 essential part. A group of strata extending over a certain 

 geographical extent, all of which contain some fossils in com- 

 mon, no matter what may be the chemical character of the 

 rock, whether it be limestone, sand, or clay, is termed a 

 geological Formation. Thus, the coal beds, with the inter- 

 vening slates and grits, and the masses of limestone between 

 which they often lie, constitute but one formation, the car- 

 boniferous formation. 



650. Among the stratified rocks, we distinguish ten prin- 

 cipal formations, each of w r hich indicates an entirely new 

 era in the earth's history ; while each of the layers com- 

 posing a formation indicates but some partial revolution. 

 Proceeding from below upwards, they are as follows, as 



